Frame Relay

History of Frame Relay


Frame Relay is a high-performance WAN protocol that operates at the physical and data link layers of the OSI reference model. Frame Relay originally was designed for use across Integrated Services Digital Network (ISDN) interfaces. Today, it is used over a variety of other network interfaces as well

Initial proposals for the standardization of Frame Relay were presented to the Consultative Committee on International Telephone and Telegraph (CCITT) in 1984. Because of a lack of interoperability and standardization frame relay did not experience large scale implementation until the 90’s.
A major development in Frame Relay's history occurred in 1990 when Cisco, Digital Equipment Corporation (DEC), Northern Telecom, and StrataCom formed a consortium to focus on Frame Relay technology development. This consortium developed a specification that conformed to the basic Frame Relay protocol that was being discussed in CCITT, but it extended the protocol with features that provide additional capabilities for complex internetworking environments. These Frame Relay extensions are referred to collectively as the Local Management Interface (LMI).
Internationally, Frame Relay was standardized by the International Telecommunication Union—Telecommunications Standards Section (ITU-T). In the United States, Frame Relay is an American National Standards Institute (ANSI) standard.









Frame Relay Overview

Devices attached to a Frame Relay WAN fall into the following two general categories:
• Data terminal equipment (DTE)
• Data circuit-terminating equipment (DCE)
DTEs generally are considered to be terminating equipment for a specific network and typically are located on the premises of a customer.
DCEs are carrier-owned internetworking devices. The purpose of DCE equipment is to provide clocking and switching services in a network, which are the devices that actually transmit data through the WAN.
Frame Relay Virtual Circuits
Frame Relay provides connection-oriented data link layer communication. This means that a defined communication exists between each pair of devices and that these connections are associated with a connection identifier. This service is implemented by using a Frame Relay virtual circuit, which is a logical connection created between two data terminal equipment (DTE) devices across a Frame Relay packet-switched network (PSN).
Virtual circuits provide a bidirectional communication path from one DTE device to another and are uniquely identified by a data-link connection identifier (DLCI). A number of virtual circuits can be multiplexed into a single physical circuit for transmission across the network. This capability often can reduce the equipment and network complexity required to connect multiple DTE devices.
A virtual circuit can pass through any number of intermediate DCE devices (switches) located within the Frame Relay PSN.
Frame Relay virtual circuits fall into two categories: switched virtual circuits (SVCs) and permanent virtual circuits (PVCs).
• Switched virtual circuits (SVCs), which are temporary connections that are created for each data transfer and then are terminated when the data transfer is complete (not a widely used connection)
• Permanent virtual circuits (PVCs), which are permanent connections
The DLCI is a value assigned to each virtual circuit and DTE device connection point in the Frame Relay WAN. Two different connections can be assigned the same value within the same Frame Relay WAN—one on each side of the virtual connection.

Review of the extensions
In 1990, Cisco Systems, StrataCom, Northern Telecom, and Digital Equipment Corporation developed a set of Frame Relay enhancements called the Local Management Interface (LMI). The LMI enhancements offer a number of features (referred to as extensions) for managing complex internetworks, including the following:
• Global addressing
• Virtual circuit status messages
• Multicasting

Inverse Arp
Frame-Relay (a Layer 2 protocol) uses Inverse-Arp to map a know Layer 2 Address (DLCI) to a unknown Layer 3 Address (for example, IP).

View example slide 4

Ethernet ARP Request knows the Layer 3 Address, and requests the Layer 2 Address (MAC), on the other hand Frame Relay Inverse ARP knows the Layer 2 Address (DLCI) and request the Layer 3 Address (Next-Hop IP Address).
Once the interface is enabled, the router will send Inverse-Arp requests out all DLCIs learned via LMI for all protocols configured on the interface.
You can rely on Inverse-Arp to map the DLCI to the IP Address of your Routers, or if you want (or the situation asks for), you can create static maps.
Frame-Relay Inverse ARP is not required on point-to-point interface, and, if needed it can be disabled with the “no frame-relay inverse arp” command under the interface you need to do it:

However, Inverse ARP Reply can NOT be disabled, that means, even if you disable your Inverse ARP in an interface, if this interface receives any Inverse ARP Request it´ll respond to the request!
We can check which type of mapping was configured with the command "show frame-relay map":
- dynamic means the mapping was done using Inverse-Arp;
- static means the mapping was done manually (by command-line-interface).


Go over configuration scenarios and then implement them!